Method and apparatus for correcting the offset induced by field effect transistor photo-conductive effects in a solid state X-ray detector

ABSTRACT

A method and apparatus for correcting the offset induced by Field Effect Transistor (FET) photo-conductive effects in solid state x-ray detectors includes dedicating rows at the beginning and end of an x-ray detector scan. The dedicated rows may be used to measure the “signal” induced by the photo-conductivity of FET switches in solid state x-ray detectors. Since the signal induced by FET photo-conductivity decays over time, the measurements taken at the beginning and end of a detector scan may be used to predict by interpolation the amount of signal contributed by photo-conductive induced offset for each row of the detector scan on a column by column basis.

CROSS REFERENCE TO RELATED APPLICATIONS (if applicable)

[0001] Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT (ifapplicable)

[0002] Not Applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention generally relates to medical diagnosticimaging systems, and in particular relates to a method and apparatus forcorrecting the digital image offset induced by Field Effect Transistor(FET) photo-conductive effects in medical imaging systems employingsolid state detectors.

[0004] X-ray imaging has long been an accepted medical diagnostic tool.X-ray imaging systems are commonly used to capture, as examples,thoracic, cervical, spinal, cranial, and abdominal images that ofteninclude information necessary for a doctor to make an accuratediagnosis. X-ray imaging systems typically include an x-ray source andan x-ray sensor. When having a thoracic x-ray image taken, for example,a patient stands with his or her chest against the x-ray sensor as anx-ray technologist positions the x-ray sensor and the x-ray source at anappropriate height. X-rays produced by the source travel through thepatient's chest, and the x-ray sensor then detects the x-ray energygenerated by the source and attenuated to various degrees by differentparts of the body. An associated control system obtains the detectedx-ray energy from the x-ray sensor and prepares a correspondingdiagnostic image on a display.

[0005] The x-ray sensor may be a conventional screen/film configuration,in which the screen converts the x-rays to light that exposes the film.The x-ray sensor may also be a solid state digital image detector.Digital detectors afford a significantly greater dynamic range thanconventional screen/film configurations.

[0006] One embodiment of a solid state digital x-ray detector may becomprised of a panel of semiconductor FETs and photodiodes. The FETs andphotodiodes in the panel are typically arranged in rows (scan lines) andcolumns (data lines). A FET controller controls the order in which theFETs are turned on and off. The FETs are typically turned on, oractivated, in rows. When the FETs are turned on, charge to establish theFET channel is drawn into the FET from both the source and the drain ofthe transistor. The source of each FET is connected to a photodiode.Each photodiode integrates the light signal emitted by the scintillatorabove it in response to the absorption of x-rays and discharges energyin proportion to the x-rays absorbed. The gates of the FETs areconnected to the FET controller. The Image Acquisition Module readssignals discharged from the panel of FETs and photodiodes. The ImageAcquisition Module converts the signals discharged from the panel ofFETs and photodiodes. The converted energy discharged by the photodiodesin the detector is used by the Image Acquisition Module to activatepixels in the displayed digital diagnostic image. The panel of FETs andphotodiodes is typically scanned by row. The corresponding pixels in thedigital diagnostic image are typically activated in rows.

[0007] The FETs in the x-ray detector act as switches to control thecharging and discharging of the photodiodes. When a FET closes, anassociated photodiode is recharged to an initial charge. While the FETsare open, the photodiodes are bombarded with x-rays. The number ofx-rays experienced by each photodiode corresponds to the x-ray dose. Thex-rays are absorbed by the scintillator above the photodiode, whichemits light and discharges the photodiodes in contact therewith. Thus,after the conclusion of the exposure, while the FETs are open, thephotodiodes retain a charge representative of the x-ray dose. When a FETis closed, a certain amount of charge is applied thereto in order tore-establish a desired charge across the photodiode. When a FET isclosed, the amount of charge required to restore the initial charge oneach photodiode is measured. The measured charge amount becomes ameasure of the x-ray dose integrated by the scintillator, with theresulting light integrated by the photodiode during the length of thex-ray exposure.

[0008] X-ray images may be used for many purposes. For instance,internal defects in a target object may be detected. Additionally,changes in internal structure or alignment may be determined.Furthermore, the image may show the presence or absence of objects inthe target. The information gained from x-ray imaging has applicationsin many fields, including medicine and manufacturing.

[0009] In any imaging system, x-ray or otherwise, image quality is ofprimary importance. In this regard, x-ray imaging systems that usedigital or solid state image detectors (“digital x-ray systems”) facecertain unique difficulties. In particular, digital x-ray systems mustmeet stringent demands on Critical to Quality (CTQ) measurements inorder to provide a usable image. CTQ measurements include imageresolution, image resolution consistency (e.g., comparing an image fromone system to another system), and image noise (artifacts, “ghosts,” ordistortions in the image). In the past, however, digital x-ray systemswere often unable to meet CTQ requirements or provide consistent imagequality. This deficiency in part may be due to process variations in thesemiconductor fabrication techniques used to manufacture solid statedigital image detectors. Additionally, the decrease in image quality maybe due to the inherent charge retention properties of semiconductormaterials used in imaging technology.

[0010] Many semiconductor devices exhibit photo-conductivecharacteristics. Photo-conductivity is an increase in electronconductivity of a material through optical (light) excitation ofelectrons in the material. Photo-conductive characteristics areexhibited by the FETs used as switches in solid state x-ray detectors.Ideally, FET switches isolate the photodiode from the electronics whichmeasure the charge restored to the photodiode. FETs exhibitingphoto-conductive characteristics do not isolate the photodiode from thesystem, when the FETs are open. Consequently, charge from multiplephotodiodes is restored simultaneously by the Image Acquisition Module.The Image Acquisition Module can not distinguish to which photodiodesthe charge is restored, which corrupts the image acquisition process.The unintended charge leakage through the FETs may produce artifacts orghost images or may add a charge offset to component values in thedigital x-ray image.

[0011] FETs and other materials made of amorphous silicon also exhibit acharacteristic referred to as charge retention. Charge retention is astructured phenomenon and may be controlled to a certain extent. Chargeretention corresponds to the phenomenon whereby not all of the chargedrawn into the FET to establish a conducting channel is forced out whenthe FET is turned off. The retained charge leaks out of the FET overtime, even after the FET is turned off, and the leaked charge from theFET adds an offset to the signal read out of the photodiodes by thex-ray control system.

[0012] The FETs in the x-ray detector exhibit charge retentioncharacteristics when voltage is applied to the FETs to read the rows ofthe x-ray detector. The detector rows are generally read in apredetermined manner, sequence, and time interval. The time interval mayvary between read operations for complete frames of the x-ray image.When a FET is closed, the charge on an associated photodiode is restoredby a charge measurement unit but the FET retains a portion of thecharge. When the FETs are opened, between read operations, a portion ofthe charge retained by the FETs leaks from the FETs to a chargemeasurement unit. The amount of charge that leaks from the FETsexponentially decays over time. The next read operation occurs beforethe entire retained charge leaks from the FETs. Consequently, the chargemeasurement unit measures during each read operation an amount of chargethat was retained by the FETs during the previous read operation.

[0013] The charge remaining on the FETs when a new read operation isinitiated is referred to as the initial charge retention. The initialcharge retention stored on multiple FETs, such as the FETs of a singlerow of column, combines to form a charge retention offset. The chargeretention offset varies based on the rate at which rows of the x-raydetector panel are read. As the interval increases between readoperations, the charge decay increases. When the panel rows are read ata constant rate, the charge retention offset builds to a steady statevalue. The steady state value for the charge retention rate representsthe point at which the panel rows are read at a rate equaling theexponential decay rate of the charge on the FETs.

[0014] If the times between frames for both the offset and x-ray imageare consistent, the effect of charge retention may be eliminated fromthe final image. In the normal process of reading a detector, the effectof retained charge may be minimized, during calibration, by simplysubtracting the results of a “dark” scan from the results of an“exposed” scan. A “dark” scan is a reading done without x-ray exposure.A “dark” scan simply activates the FETs on the x-ray detector panel.Thus, a “dark” scan may determine the charge retention characteristicsexhibited by the FETs activated to read the x-ray detector. Bysubtracting the “dark” scan from the actual “exposed” scan of a desiredobject, the charge retention effects may be eliminated.

[0015] During an x-ray exposure, a similar phenomenon occurs wherebycharge is generated in the FET as a result of the FET photo-conductivecharacteristics. When the FETs are turned off at the end of theexposure, the additional charge also leaks out and adds to the readsignal in a manner analogous to charge retention. However, theadditional charge cannot be removed because the additional chargeresulting from the FET photo-conductive characteristics relates to thex-rays bombarding the x-ray detector. Thus, the additional chargeresulting from the FET photo-conductive characteristics is notpredictable or nor is it reproducible in a “dark” image where no x-raysare transmitted. The number of FETs that photo-conduct and the amount ofcharge conducted by the FETs are dependent upon the amount of x-rayexposure and the object imaged, as well as upon the individualproperties of each FET. Since a solid state x-ray detector is structuredalong rows (scan lines) and columns (data lines), the excess charge inthe FETs may result in structured image artifacts or offsets whichcannot be corrected by contrasting the “exposed” image with a “dark”image.

[0016] Photo-conductivity is not as structured as charge retention.First, when a FET in the x-ray detector is turned on to be read, the FETis always turned on with the same voltage. With the photo-conductiveeffect, the “amount” that the FET is turned on is determined by theintensity of the light reaching a given FET. The light reaching the FETsmay vary among a wide range of intensities for all of the FETs on thex-ray detector. Second, regardless of how strongly each FET is affectedby photo-conductivity (due to the light intensity at each FET), all ofthe FETs will be affected simultaneously. Charge retention onlystimulates one FET in any given column at a time. Therefore,photo-conductivity is much more unpredictable and is uncorrectable by asimple image subtraction method.

[0017] As noted above, the characteristics of digital image detectorsinherently vary. Although there is a need to provide consistent imagequality (and in particular, image resolution) within and across multiplemedical diagnostic imaging systems, there has been in the past noautomated technique for providing such consistency. Furthermore, thestringent CTQ requirements may result in low acceptable yields fordigital image detectors which are then destroyed, or, at best, deemedunusable for medical diagnostic systems. Consequently, time, money, andresources are wasted.

[0018] Thus, a need exists for a method and apparatus for correcting theoffset induced by Field Effect Transistor photo-conductive effects in asolid state x-ray detector.

BRIEF SUMMARY OF THE INVENTION

[0019] A preferred embodiment of the present invention provides a methodand apparatus for correcting the digital image offset induced by FieldEffect Transistor (FET) photo-conductive effects in solid state x-raydetectors. The method and apparatus include adding one or more rows toboth the beginning and end of a normal x-ray detector scan area. Theadditional rows may be outside the physical image area of a solid statex-ray detector. The additional rows then may be used to measure the“signal” induced by the photo-conductivity of FET switches in a solidstate x-ray detector. The signal may be measured at both the beginningand end of a detector scan. Since the signal induced by FETphoto-conductivity decays over time, the measurements taken at thebeginning and end of a detector scan may be used to predict byinterpolation the amount of signal contributed by photo-conductiveinduced offset for each row of the detector scan on a column by columnbasis.

[0020] An alternative preferred embodiment may use an existing solidstate x-ray detector scan area and simply not activate one or more rowsat the beginning and end of the x-ray detector scan area. Thisembodiment may reduce the image area covered by the scan.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 illustrates a preferred embodiment of a general medicaldiagnostic imaging system used in connection with the present invention.

[0022]FIG. 2 illustrates a flow diagram of a preferred embodiment forcorrecting the offset induced by FET (Field Effect Transistor)photo-conductive effects in a solid state x-ray detector.

[0023]FIG. 3 shows a wave diagram demonstrating a method for acquiringan image according to a preferred embodiment of the present invention.

[0024]FIG. 4 illustrates a preferred embodiment of a solid state x-raydetector.

[0025]FIG. 5 illustrates a preferred embodiment of a solid state x-raydetector scan area with two additional rows for offset correction at thebeginning of the x-ray detector scan area and two additional rows foroffset correction at the end of the x-ray detector scan area.

[0026]FIG. 6 illustrates a preferred embodiment of a solid state x-raydetector scan area with two rows dedicated for offset correction at thebeginning of the x-ray detector scan area and two rows dedicated foroffset correction at the end of the x-ray detector scan area.

[0027]FIG. 7 illustrates a preferred embodiment of a solid state x-raydetector scan.

DETAILED DESCRIPTION OF THE INVENTION

[0028]FIG. 1 illustrates a preferred embodiment of a medical diagnosticimaging system 100 used in accordance with the present invention. Themedical diagnostic imaging system 100 includes a plurality ofsubsystems. For the purposes of illustration only, the medicaldiagnostic imaging system is described as an x-ray system. The medicaldiagnostic imaging system 100 includes subsystems, such as an x-raydetector 110, an x-ray detector scan area 1115, an x-ray source 120, anda patient 130. The medical diagnostic imaging system 100 also includesan image acquisition module 140 and an image adjustment module 150.

[0029] The patient 130 is positioned in the medical diagnostic imagingsystem 100. In one exemplary system, an x-ray source 120 is positionedabove the patient 130. The x-ray detector 110 is positioned below thepatient 130. X-rays are transmitted from the x-ray source 120 throughthe patient 130 to the x-ray detector 110 and the x-ray detector scanarea 115.

[0030]FIG. 4 illustrates a preferred embodiment of a solid state x-raydetector scan area 115 within an x-ray detector 110. The x-ray detectorscan area 115 is comprised of cells 410 corresponding to pixels in anx-ray image. Each cell 410 typically comprises a photodiode and a FieldEffect Transistor (FET). The cells 410 may be arranged in columns (datalines) 420 and rows (scan lines) 430. The cells 410 are activated by row430 and by column 420. One or more cells 410 are uniquely mapped to oneor more pixels in an x-ray image. The pixels are activated to producethe desired digital x-ray image of the patient 130.

[0031]FIG. 7 illustrates a lower-level view of a preferred embodiment ofa solid state x-ray detector scan area 115 within an x-ray detector 110.The x-ray detector scan area 115 is comprised of cells 710 comprising aphotodiode 720 and a Field Effect Transistor (FET) 730. Leads 740connect the cells 710 to the image acquisition module 140.

[0032] The image acquisition module 140 acquires an x-ray image from thex-ray detector scan area 115. In a preferred embodiment, the imageacquisition module 140 may acquire an image from an area, an exposeddetector section, larger than the normal patient subsection of the x-raydetector scan area 115. In a preferred embodiment, shown in FIG. 5, thex-ray detector scan area 115 may be enlarged by scanning additional rows510 before the beginning of the x-ray detector scan area 115 or scanningadditional rows 510 after the end of the x-ray detector scan area 115 toform an enlarged x-ray detector scan area 115. The number of rows 510may vary. Also, the rows 510 may be located along one of both sides ofthe x-ray detector scan area 115, in addition to or in place of beinglocated before and after the x-ray detector scan area 115. The imageacquisition module 140 may acquire the image from the enlarged x-raydetector scan area 115.

[0033] In another preferred embodiment, shown in FIG. 6, the x-raydetector scan area 115 may be reduced by one or more rows 610 at thebeginning of the x-ray detector scan area 115 and one or more rows 610at the end of the x-ray detector scan area 115 and/or one or more rowsalong either side of the x-ray detector scan area 115. The rowsdedicated in the normal x-ray detector scan area 115 may be used foroffset correction in place of the additional rows 510 added in anotherpreferred embodiment. The image acquisition module 140 may acquire anx-ray image from the x-ray detector scan area 115.

[0034] The image acquisition module 140 may acquire an x-ray image fromthe x-ray detector scan area 115 by receiving a signal from the leads740 from the cells 410, 710 in the x-ray detector scan area 115. Thesignal from the leads 740 may be generated by charge stored in thephotodiodes 720. The charge stored in the photodiodes 720 may resultfrom absorption of x-ray energy by the photodiodes 720. The FETs 730allow the charge stored in the photodiodes 720 to travel as a signalthrough the leads 740. The FETs 730 may be actuated by the imageacquisition module 140. The signal received by the image acquisitionmodule 140 through the leads 740 may include an offset produced by thecharge retention characteristics and photo-conductive effects of theFETs 730.

[0035] The image adjustment module 150 receives the acquired image fromthe image acquisition module 140. The image adjustment module 150corrects the offset induced in the image by the x-ray detector 110. Theoffset in the x-ray image may be induced by the photo-conductive and/orcharge retention properties of the FETs (Field Effect Transistors) 730in the x-ray detector 110. In a preferred embodiment, the chargeretention offset from the FETs 730 may be eliminated using a “dark”image containing the charge leakage caused by charge retention in theFETs. In a preferred embodiment, the additional rows scanned at thebeginning and end of the x-ray detector scan area 115 are utilized bythe image adjustment module 150 to correct the offset induced by FETphoto-conductive effects in the x-ray image. In an alternative preferredembodiment, the rows dedicated at the beginning and end of the normalx-ray detector scan area 115 are utilized by the image adjustment module150 to correct the offset induced by FET photo-conductive effects in thex-ray image.

[0036] Turning now to FIG. 2, the figure illustrates a flow diagram 200for a preferred embodiment for correcting the offset induced in amedical diagnostic imaging system according to the present invention. Instep 210, the image acquisition module 140 acquires a “dark” image fromthe x-ray detector scan area 115. A “dark” image is obtained from areading done without an x-ray exposure. A scan for a “dark” imageactivates the FETs 730 in the x-ray detector scan area 115 and measuresretained charge leakage from the FETs 730. Thus, a “dark” image may beused to determine the charge retention offset produced by the FETs 730activated to read the x-ray detector scan area 115.

[0037] In step 220, the image acquisition module 140 acquires an x-rayimage from the x-ray detector scan area 115. The image is offset byexcess charge from sources such as the photo-conductive effects andcharge retention characteristics of FETs 730 comprising the solid statex-ray detector 110. The x-ray image is read row by row by the imageacquisition module 140 from the x-ray detector scan area 115 using leads740 from cells 710 in the x-ray detector scan area 115. In a preferredembodiment of the present invention, the image acquisition module 140acquires two additional rows 510 at the beginning of the image scan andalso acquires two additional rows 510 at the end of the image scan. Theadditional rows 510 do not represent the object being scanned. Theadditional rows 510 indicate the offset charge “signal” that is inducedby FET photo-conductive effects. In another preferred embodiment of thepresent invention, the image acquisition module 140 dedicates two rows610 at the beginning of the x-ray detector scan area 115 and two rows610 at the end of the x-ray detector scan area 115 to photo-conductivitymeasurement, thus reducing the overall size of the acquired image.

[0038] During operation, the image acquisition module 140 performsconsecutive or successive scans (read operations) of each row 430 ofcells 410, 710 in the x-ray detector scan area 115. First, the imageacquisition module 140 scans one or more rows 510, 610 outside (e.g.,before) the scanned image area of the x-ray detector scan area 115. Theimage acquisition module 140 acquires photo-conductive offset data fromthe rows 510, 610 scanned outside the scanned image area of the x-raydetector scan area 115. The image acquisition module 140 then performs arow by row scan of each row 430 in the scanned image area of the x-raydetector scan area 115. During the row by row scan of each row 430 inthe scanned image area of the x-ray detector scan area 115, the imageacquisition module 140 obtains exposure data for each cell 410, 710 inthe x-ray detector scan area 115. The image acquisition module 140 thenmay scan one or more other rows 510, 610 outside (e.g., after) thescanned image area of the x-ray detector scan area 115. The imageacquisition module 140 acquires photo-conductive offset data from therows 510, 610 scanned outside the scanned image area of the x-raydetector scan area 115.

[0039] In step 230, the image adjustment module 150 receives x-ray imagedata from the image acquisition module 140. The image includes theadditional rows dedicated to offset correction at the beginning and endof the image scan. The image adjustment module 150 analyzes the image ona pixel by pixel basis, according to row and column. In step 240, theimage adjustment module 150 calculates the image data value for a pixel410 in the image. For each pixel, the image data value (ID) is equal tothe exposure data value (ED) from the image minus the charge retentionoffset data value (CR) from the “dark” image minus the calculatedphotoconductive offset data value (PC) from the offset correction rows510, 610 (or ID_(ij)=ED_(ij)−CR_(ij)−PC_(ij)). In the calculation, irepresents the row 430 index into the image and j represents the column420 index into the image. The calculated photoconductive offset datavalue for each pixel 410 i in a given column 420 j is((R_(N)−R_(i))/R_(N))*½((R−2)_(e)+(R−1)_(e)−(R_(N)+1)_(e)−(R_(N)+2)_(e)−(R−2)_(o)+(R_(N)+1)_(o)+(R_(N)+2)_(o)), where (R−2)_(e)represents the signal measured in the expose frame for pixel (−2,j), andR_(N) and R_(i) are row numbers. The subscript “e” refers to the exposeframe, and the subscript “o” refers to the offset frame, as depicted inFIG. 3. The resulting image data values for each pixel in the image maybe used to generate a digital display.

[0040] Thus, the present invention provides a solution to what hasbecome a serious degradation issue for solid state x-ray detectors. Themethod and apparatus for measuring and correcting the offset induced byphoto-conductive FETs in a solid state x-ray detector may improve thedesign of new medical diagnostic imaging systems and may preserveexisting medical diagnostic imaging systems through offset correction.The present invention may be easily implemented and does not necessarilyrequire a change to existing hardware.

[0041] While the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for generating a medical diagnosticimage acquired by a detector in a medical diagnostic imaging systemcomprising: exposing a detector to an energy source to form an exposeddetector section including an exposed patient subsection; measuring atleast first and second data sets generated by the detector, one of saidfirst and second data sets being representative of at least a portion ofsaid exposed detector section outside said exposed patient subsectionand one of said first and second data sets being representative of atleast a portion of said exposed patient image; and generating a medicaldiagnostic image based on said exposed patient subsection and a relationbetween said first and second data sets.
 2. The method of claim 1wherein said step of exposing a detector to an energy source comprisesexposing said detector to x-ray energy.
 3. The method of claim 1 whereinsaid first and second data sets comprise an exposure data set and acorrection data set.
 4. The method of claim 3 wherein said step ofmeasuring at least first and second data sets comprises measuring saidat least a portion of said exposed detector section outside said exposedpatient image for said correction data set and measuring said at least aportion of said exposed patient image for said exposure data set.
 5. Themethod of claim 3 wherein said step of generating said medicaldiagnostic image comprises subtracting a value from said correction dataset from a corresponding value in said exposure data set.
 6. The methodof claim 1 wherein said step of generating said medical diagnostic imagecomprises subtracting a value from said first data set from acorresponding value in said second data set.
 7. The method of claim 1wherein said step of generating a medical diagnostic image comprisesactivating pixels in a digital display according to said measurements insaid first and second data sets.
 8. The method of claim 3 wherein saidcorrection data set includes a measure of Field Effect Transistorphoto-conductive effects.
 9. The method of claim 1 wherein said step ofmeasuring at least first and second data sets comprises said first dataset being representative of at least a portion of said exposed detectorsection outside said exposed patient image and said second data setbeing representative of at least a portion of said exposed patientimage.
 10. The method of claim 9 further comprising measuring at least athird data set being representative of at least a second portion of saidexposed detector section outside said exposed patient image.
 11. Adetector subsystem for acquiring an image comprising: a panel beingexposed to energy representative of an object and energy outside of saidobject, said panel formed of an array of cells detecting discreteamounts of energy; and a scanner for reading data sets, each of which isrepresentative of an amount of energy stored by a cell; said scannerreading at least first and second data sets before and after said panelbeing exposed to said energy; one of said at least first and second datasets read from at least a portion of said object and one of said atleast first and second data sets read from at least a portion of saidoutside of said object; said scanner producing a detector output basedon a relation between the first and second data sets.
 12. The subsystemof claim 11 wherein said array of cells comprises: an array ofphotodiodes storing charge representative of said discrete amounts ofenergy; and an array of Field Effect Transistors switchablyinterconnecting said photodiodes and said scanner.
 13. The subsystem ofclaim 11 wherein said first and second data sets comprise an exposuredata set and a correction data set.
 14. The subsystem of claim 13wherein said correction data set includes Field Effect Transistorphoto-conductive effects.
 15. The subsystem of claim 13 wherein saidscanner reads said first data set from at least a portion of said panelwith said object for said exposure data set and said scanner reads saidsecond data set from at least a portion of said panel outside saidobject for said calibration data set.
 16. A medical diagnostic imagingsystem, comprising: a detector for detecting an energy pattern emanatingfrom a patient; said detector having an array of discrete collectingelements storing charge representative of an amount of detected energyboth from said patient and outside said patient; an image acquisitionmodule scanning a charge stored on said collecting elements; and saidimage acquisition module scanning said collecting elements during afirst pass to obtain data representative of the intrinsic energycharacteristic from an unexposed detector and during a second pass toobtain both exposure data representative of an energy pattern from saidpatient and correction data representative of an energy pattern of saiddetector from outside said patient;
 17. The system of claim 16 furthercomprising: an image adjustment module correcting said exposure datausing said correction data to minimize the effect of said energycharacteristic of said detector.
 18. The system of claim 16 wherein saiddetector further comprises: an array of Field Effect Transistorsswitchably interconnecting said collecting elements and said imageacquisition module.
 19. The system of claim 18 wherein said energycharacteristic of said detector includes Field Effect Transistorphoto-conductive effects.
 20. The system of claim 16 wherein saidcollecting elements comprise photodiodes.
 21. The system of claim 16wherein said energy pattern is an x-ray energy pattern.